(1) Field of the Invention
This invention relates to methods and apparatus for applying liquid coolant to, and subsequently removing the coolant from, metal strip advancing in a continuous line. Preferably, although not exclusively, the invention is directed to the cooling of metal strip in single-stand and multi-stand cold rolling mills. Still more particularly, the invention is concerned with methods and apparatus for liquid cooling of metal strip, such as aluminum strip.
(2) Description of the Related Art
In the processing of metal sheet such as aluminum strip (the term “aluminum” being used herein to refer to aluminum-based alloys as well as pure aluminum metal), such metal sheet is often cooled by application of a coolant liquid (e.g. water) followed shortly thereafter by the removal of the coolant liquid before further processing or coiling takes place. For example, coated metal sheet may be cooled following the application and heat-curing of a layer of coating lacquer, or hot-rolled aluminum strip may be cooled before it is coiled at the end of a hot roll line or in a separate coil-to-coil operation. In such cases, the coolant liquid is often applied to just one side of the metal sheet and steps may be taken to avoid contact of the coolant liquid with the other side of the sheet if such contact would cause damage (e.g. to a coating layer) or undesirable marking or staining.
A further important situation where such cooling is carried out is during the reduction in thickness of metal (especially aluminum) strip by cold working in one or a tandem succession of roll stands each typically including upper and lower work rolls (between which the strip passes) and upper and lower backup rolls respectively above and below (and in contact with) the upper and lower work rolls. The strip to be reduced in thickness is paid out from a coil at the upstream end of the cold rolling line, and after passage through the roll stand or stands, is rewound into a coil at the downstream end of the line, the cold-rolling operation being essentially continuous. Unavoidably, the cold working of the strip as it passes through the nip of each roll stand is accompanied by some elevation of strip temperature. In a single-stand mill, this temperature rise is usually not troublesome provided the strip enters the mill near room temperature. In a multi-stand tandem mill, however, the increases in strip temperature at the several roll stands are cumulative, with the result that the intermediate and/or exit temperature of the strip from the mill may exceed acceptable limits, even with entry at room temperature. For example, computer model analysis of a three-stand mill indicates that the strip exit temperature can approach a value as high as 300° C., depending primarily on the particular alloy being rolled, the extent of the reductions to which the strip is subjected in the mill, and the rolling conditions. On the other hand, considerations related to process reliability, such as the avoidance of strip breaks, and metallurgical and mechanical considerations related to product performance, require that the exit or coiling temperature of cold-rolled aluminum strip be kept usually between 100° and 180° C., depending on the product, with a typical limiting value being around 150° C. Moreover, in the case of some products, it would be highly advantageous to control the coiling temperature of cold-rolled strip within some predetermined range for maximum efficiency and benefit in subsequent process steps.
At the same time, as the cold-rolled strip is cooled, it is important that the cooling operation not adversely affect other aspects of product quality. For example, while water is a preferred liquid coolant from the standpoint of cost and effectiveness, the presence of water may impair the performance of rolling lubricant at the roll stands and, if the strip is aluminum or other water-stainable metal, residual water in the rewound coil may cause unacceptable surface staining.
U.S. Pat. No. 5,701,775 which issued to Sivilotti et al. on Dec. 30, 1997 addresses these issues and provides a process and apparatus of cooling horizontally-advancing metal strips by applying upward jets of liquid coolant only to the lower surface of the strip and then removing the coolant from the lower surface by directing liquid knives against the lower surface of the strip. Recognizing that any given cold-rolling mill is usually employed at different times to roll metal strips of various different widths, arrays of laterally-movable overlapping metal shutters were provided along and slightly below each side of the strip path for deflecting upward jets of coolant positioned beyond the lateral edges of a particular strip undergoing a rolling operation. The shutters thereby prevent the cooling jets from splashing onto the upper surface of the strip and causing problems as indicated above.
While this patented solution is quite effective, experience showed that some coolant could still bypass the shutters and contact the upper surface of the strip. To prevent this, it has become usual to provide vertical brushes mounted on the upper side of the movable shutters closest to the strip. As the strip advances, the brushes push up against the underside of the strip at the edges and are slightly, and sometimes severely, splayed out under compression, thereby closing any gap between the shutters and the edges of the strip to prevent penetration of liquid. This solution also has its problems, however, in that it is not always possible to position the brushes exactly at the edges of the strip, especially if the width of the strip varies somewhat at different longitudinal positions. This can allow liquid coolant to extend onto the upper surface of the strip and/or can cause uneven cooling at the edge of the strip, thereby resulting in edge distortion (e.g. a wrinkling or buckling of the strip at the edges). There is therefore a need for further improvements of the method and apparatus.